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Should I log flight time as a passenger?
If I only have a private pilot license, and am not the pilot in command, should I log my flight time?
If so, what are the limits of this? i.e. is that as true in the backseat as it is in the front seat? Must I have that type rating?

No, you should not*.
In my opinion, you should limit your logbook to records of your own flight experience. This would include any time spent training, as PIC, or as required crew.
The time you can log as PIC is that which you spend as the sole manipulator of the controls, the sole occupant, or as the pilot in command when the required crew is >1. Here's an article from an FAA FSDO describing the logging of PIC time in more detail. Rod Machado has some good coverage of this as well.
If you're just hanging out in the airplane, it doesn't matter if you're in the front seat or the back seat. If you're receiving instruction, you should get an entry and a logbook endorsement from the instructor you're flying with. Most General Aviation aircraft don't require a type rating, so I'm unsure what the context of your final question is.

* It's worth noting that you can technically put whatever you want in your logbook, though it's not a great idea; when it comes time to fill out an FAA 8710 form for later ratings, it can get confusing. In my opinion, you still shouldn't log any time, but if you're just wanting to record flights that you took with friends, that's fine. As an instructor who's had to slog through some weird logbooks, I'd really, really encourage you to track that elsewhere, though. Your logbook is exactly that - it's your logbook, for the logging of your flight time, and a record of your flight experience and training.

Riffing on the point that you can technically put whatever you want into your logbook, it's worth noting that technically, any flight that starts at one airport and ends at another is a cross-country flight, even if they're only a few miles apart. However, for the purposes of getting certain licenses and ratings, only flights longer than 50nm can be counted. This can make a hash out of figuring out your cross-country time when applying for a new rating. I think it might simply be better not not even log the short cross-countries.

And riffing on that: the famous Dick Rutan/Deana Yeager round-the-world flight couldn't be logged as cross-country time because they landed at the same airport they took off from.

If you're looking to maintain a record of non flying time for whatever reason, just keep a second log book with those entries. It doesn't seem to be worth the confusion it could create.

@Kyle Another option is to use a "spare" column in your logbook for passenger/non-PIC/"just having fun" time, and just not add it to "total duration of flight" or similar columns.

Yes, log it.
Your logbook is your record of your flight experience. Use a line in your logbook to remember a flight with a friend or a warbird flight you took at an air show. If I'm a passenger in the right seat of a small aircraft and I'm paying attention I am probably learning something. If I'm currently working towards my instrument rating and I'm sitting in the backseat of a friend's instrument rating lesson then I'm almost certainly learning things that I wouldn't get while actually flying the plane. Record those experiences for yourself.
However, the numbers columns for that entry are going to be empty. No landings, no flight time, none of that. You wouldn't want to accidentally use those flights for currency or ratings. AOPA has a good article interpreting the FARs rules on logging time.
But you can use an empty column for passenger numbers or just use your comments section.

I agree completely! I don't do it myself, but this is a totally valid answer for those who plan to do so.

I love this answer. As a PPL student currently (almost done!) I can say that when I've gone up in the right seat with a non-CFI friend acting as PIC, even though I'm not required crew and not receiving qualified instruction, I'm learning and benefiting from the experience so I want to log it because that experience sets me apart from the next guy who doesn't have that experience.
To avoid inadvertently counting those hours and landings for recency/currency, it does make sense not to put entries in those columns.

Attempting to follow the advice in this answer is not allowed under the FARs.  See my answer below.

I believe the FAA would call this "falsifying logbook entries" and if you get caught any potential aviation career is in jeopardy... @hemp

That makes no sense, logically or per the FARs. What goes in your logbook is up to you / it is your logbook, not the FAAs. Their only concern is that you can back up the hours and endorsements you claim to have, there are no specifications stating what you can't put in a logbook. You can write short stories in it if you want to. Leave the hour columns blank for those flights where you weren't required crew or giving/receiving instruction and it's all gravy.

That said, if you do intend to make a career out of flying then you should treat everything in the aviation scope as a professional would. A professional would not write short stories in their logbook. What is permitted and what is wise are not always the same. For me, I have a lovely career outside of aviation and no intention of changing course, so my logbook is primarily kept for nostalgia and secondarily to prove I'm airworthy.

Adding to what egid said, you can only log flight time as a crew member if you're filling a role as a required crew member.
There was an enforcement action many years ago against a pilot who was flying right seat in a small cargo plane because his employer's rules required two people in the cockpit. He figured that since his employer required him to be there, it made him a required crew member and so he logged the time.
However, the FAA regs did not require him to be there, so in their view, he was not a required crew member and they took action against him for falsifying his logs.

Please note that ive removed the extra question from your answer - it is kinda fun, but not the StackExchange way. Answers should only be answers.

Fair enough. I'll ask in a comment then: when could all three people in a C172 log PIC time at the same time?

To answer the original question no, you cannot log flight time as a passenger in an aircraft.
AOPA's website has a pretty good answer.
https://www.aopa.org/training-and-safety/learn-to-fly/logging-cross-country-time/logbooks-and-logging-time

The PIC is, by Federal Aviation Regulations (FARs), responsible for the safe operation of the flight (FAR 1.1, 91.3). At any given time, there can only be one acting PIC on a flight, no matter how many pilots are on board the aircraft. To legally act as PIC, a private, commercial, and airline transport pilot must have a current medical certificate and have all required endorsements, ratings, and recency of experience for the type of aircraft being flown and the flight conditions under which the flight is conducted (FAR 61.3, 61.31, 61.56, 61.57). Sport pilots can act as PIC with a valid and current driver's license in lieu of the FAA medical (FAR 61.23). Before a flight is initiated, an agreement should be made to determine who is to be acting as PIC.

So you'll have to hold a current medical certificate for the type of flight operation being undertaken, the appropriate pilot certificate for type of flight operation being undertaken, and have sole manipulation of the flight controls of an aircraft you are properly endoresd or type-rated for.

OK, now I'm confused. I thought if e.g. you were a licensed pilot receiving instruction, both you and the instructor would log PIC time. You, as the sole manipulator of the controls, and the instructor as the person responsible for the flight.

Yes, join a frequent flyer program and log "passenger miles". This will make you a better passenger with additional benefits. Of course, these  logs document the passenger loyalty, not some needed experience.

Should you log flight time as a passenger?
Pro:

Logbooks represent many things, including a record of time and training to be used toward future certificates and ratings, as well as currency to comply with various regulations.
What you write in your log book is up to you as long as you record the flight information for those flights needed to substantiate your time

Contra:

It is important that airmen are aware of regulations related to logbook records to avoid undue fines, or even suspension of their certificate/s!
you cannot log flight time as a passenger in an aircraft.

I know this is an old question but anyway:
In previous answers it has clearly been stated, that one can not log flight time as a passeger.
What puzzles me is why would one do that anyway? Passenger does not fly the plane, but merely observes what is going on. Two very different things.
Yes some things can be learned just by looking at how other people do them, but flying is fundamentaly a multitasking excercise (although multitasking does actually not exist at all), and by simply looking at someone operating a plane, you get a dangerously simplified picture of what it's all about. Even if you already had a license. So there is actual danger in logging passenger time (or even just flights):
You will build yourself a false sense of competence. The only hours and minutes that matter are the ones that you fly the plane.

A friend encouraged me to log it after I sat in the right seat while he flew IFR and I was not rated. That didn’t seem right, so I wanted to know what the limits were. Agreed it’s a bad idea.

@Joe - You can log it as a private pilot if your friend is flying IFR in VMC while under the hood/foggles. Though, one of you has to be instrument rated to file an IFR flight plan. Since it’s VMC, a flight plan is not mandatory (in the US).

No. You're only supposed to log time when you are "manipulating the controls" i.e., PIC/SIC or as a rated crewmember. Safety pilots and many others can log time without touching the controls, but as a passenger, you have no role in the operation of the aircraft, and therefore cannot log time.

At least in the US, there are several ways to log PIC/SIC time without ever touching the controls.


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Can a colour blind person pursue a career in Commercial Flying?
I have been an ardent aviation geek since childhood and always wanted to be flying commercially. But sometime later during a medical examination through Ishihara plates test I learned that I am color blind. I have been reading quite a lot about colour blind people are considered to be medically unfit to fly. So I just wanted to confirm if this is true or should I go and take some additional test before I let go of my dream.

Are you red/green, blue/yellow, or total?

Also: Why do some air forces no allow color blind pilots.

If you want details on the regulations or specific medical requirements then you should mention which country or jurisdiction you're interested in. But there are different types of color blindness and it may not be possible to give a definite answer; only an aviation medical examiner can give you precise information about your personal situation.

Note that the first linked question and answer are written by retired colour-blind B747 captain. And it states the condition under which it is possible.

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what are the advantages and disadvantages of tilt rotor vs ducted jet engine for a VTOL aircraft?
Please could someone summarize the main advantages and disadvantages of a tilt-rotor (the V22 Osprey) vs jet (F35) system for VTOL. I have been trying to look into this but found details very sparse and not in comparison to each other.
Among other advantages and disadvantages I am very interested in

speed
energy efficiency
ease of maintenance and why.

Well, engines and rotors obey the same laws regarding induced drag (induced power in this case) as wings: larger fan/propeller/rotor is more efficient, but limited to lower speeds. And from that, the advantages should be obvious:

Tiltrotors (V-22, AW609) are only a little less efficient in hover than normal helicopters, but they can only go around 300 knots (both aircraft have max speed quoted at 275 knots); above that, the tips would become supersonic and generate too much drag.
These can be built with engines similar to what a comparable fixed-wing turboprop aircraft would need and can cary comparable payloads with only slightly higher fuel burn. The AW609 is a product for civilian market aiming at charter operators supplying oil platforms and similar remote stations where it should be more efficient than helicopter.
The main disadvantage is that they are more complex and therefore more expensive to build and maintain.

Tiltjets/vectored thrust jets (VJ-101, Harrier, Yak-38, F-35B) are horribly inefficient in hover, but they can go fast.
Jet engine for thrust/weight ratio 1 (needed for hover) takes up most of the aircraft, leaving very little room for payload. Such design and thrust/weight ratio is common in fighters, but out of question for anything else.
This configuration also consumes a lot of fuel for take-off and landing. Absolutely no way would anything like that be competitive for commercial operation. However, the military is willing to pay it if they can have a fighter jet that can be deployed from an assault ship. And even they prefer conventional aircraft when possible.
This is really niche design, but one that offers enough tactical advantage that the Marines and the Royal Navy want it (US Navy will use the conventional F-35C with CATOBAR, Royal Navy decided it will be cheaper to use the STOVL F-35B version than upgrading the carrier they are building to CATOBAR).

I can't really compare maintenance. Obviously both are more complex than conventional aircraft and even than helicopters. But also, anything military is usually much more maintenance intensive than civilian designs, because military usually wants to squeeze out every bit of performance at the cost of simplicity.

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What are the radio characteristics of communications between tower and aircraft?
What type of antenna are used today for tower-aircraft comms? Are they directional or omni-directional?
What's the range of data rate (in kbps) that these antennas can provide?
Also, is there any safety issues related to power transmission between tower and aircraft? What's the power limit for this transmission? I know that the tower controls the plane until the cruising altitude and after that it hands it over to enroute. Before enroute takes control, how does the tower ensure that the transmit tower is not too high while ensuring it can still reach the plane?

The tower does not control until cruising altitude. Shortly after takeoff they are handed off to either a TRACON or an en route controller. TRACON can be located at the airport, but usually is not.

Going the other direction, radio in the aircraft have decent output power. For example, the Avidyne IF540 GPS Moving Map/Nav/Com is spec'ed for:
VHF COM Power Output
10 Watts nominal (12V power source)
and
Optional 16 Watt (for 28vdc installations only)

What type of antennas are used today for tower-aircraft comms? Are they directional or omni-directional?

The antennas are omni-directional however on the aircraft the mounting is important to maintain line of site and the FAA covers it in this AC. This is in reference to <12,500LB aircraft but the logic applies to all planes.

d. VHF Com 1 should be mounted on the top of the aircraft since this
  will provide the best unobstructed location. VHF Com 2 can also be
  mounted on the top, provided there is at least 1/2 wavelength (of the
  antenna operating frequency) distance available between antennas.
e. If Com 2 is mounted under the aircraft, a bent whip may be required
  to provide ground clearance. Bent whips may not provide the best
  performance because of proximity to the aircraft skin. Signal
  reflection and obstruction is more of a problem with such locations.
  Extension of the landing gear or flaps may also impact belly mounted
  antenna performance.
f. Antennas need to “see” with a direct lineof-site to the source.
  Antenna patterns can be disrupted by landing gear or vertical
  stabilizers, for example. When mounting antennas, try to locate them
  in areas where line-of-sight view is not obstructed.

They are omni-directional:  Anyone, in any direction, can hear the broadcast.
They are analog (non-digital), AM signals, so there are no bits, and thinking in terms of kbps or data-rate doesn't make sense.
These have similar transmission power to many AM radio stations.  Do you have safety concerns about local AM radio stations that broadcast baseball games?
Finally, the tower does not "control" the airplane.  The pilot on-board is the one in control.  The tower may simply give traffic directions, such as what heading to take, or what altitude is appropriate.

"These have similar transmission power to many AM radio stations." Are you sure? I don't know about US, but broadcast radio stations tend to have quite high power outputs. A more apt comparison would probably be a cell phone, at least in terms of power output...

"Although most car phones have a transmitter power of 3 watts, a handheld cell phone operates on about 0.75 to 1 watt of power." ... "Many cell phones have two signal strengths: 0.6 watts and 3 watts (for comparison, most CB radios transmit at 4 watts)."
Power of a Cell Phone Transmitter - The Physics Factbook
https://hypertextbook.com/facts/2006/EbruBek.shtml So personnal handheld phones would seem to be pretty lower power.  The cell towers are likely higher power.

Cell Tower Power Output
Although the FCC permits an effective radiated power (ERP) of up to 500 watts per channel (depending on the tower height), the majority of cellular or PCS cell sites in urban and suburban areas operate at an ERP of 100 watts per channel or less.
Human Exposure to Radio Frequency Fields: Guidelines for Cellular Antenna Sites
https://www.fcc.gov/consumers/guides/human-exposure-radio-frequency-fields-guidelines-cellular-and-pcs-sites

It would seem the ATC outputs pretty low power!
This is from a General Dynamics Air Traffic Control radio data sheet
RF Output Power:

Low Power Transmitter 2-12 Watts with co-site filter,
  2-20 Watts with out co-site filter
  High Power Transmitter 12-35 Watts with co-site
  filter, 12-50 without filter

https://gdmissionsystems.com/-/media/General-Dynamics/C4ISR-Technologies/Radios/PDF/radios-cm300-350-v2-vhf-uhf-air-traffic-control-transmitter-datasheet.ashx?la=en&hash=357448B534D17E0AD21A91485D60C3C110972654
I don't know if this would be a radio for an airport control tower, which talks to in a range of about 10-15 miles, or an Approach radio which talks to you over a wider range, like 30 miles or more.
The datasheet does say  "these rack
mounted transmitter and receiver systems are specifically designed to
meet the dynamic mission requirements of air traffic control centers,
commercial airports," so perhaps both?
When I flew from just west of Boston south to Nantucket, a distance of 98 nautical miles as the crow flies, I started with Boston Approach for flight following, got switched to a 2nd Boston Approach, got switched to Providence Approach (I flew a little more southerly to go over Martha's Vineyard and less over the ocean), then a 3rd Boston Approach, and finally Nantucket Tower when about 8-10 miles from the airport.  Same going back.

The increase in pure path loss going from 10 miles to 30 miles is something like 7-8 dB for a line-of-sight transmission. While significant in absolute terms, that's not a huge difference in practice.

For VHF/UHF airband, 10-15W is enough to reach pretty much any aircraft above the horizon; if they're below the horizon, they'd be talking to a different station--or using HF, for oceanic flights.  You can't directly compare to terrestrial radio power levels, which will need far, far more power for the same distances.

ATC uses multiple kinds of antennas depending on the use case. The antenna that makes sense for a tower might not make sense for a TRACON communicating with a specific sector that is not directly over the antenna.
Note that "omnidirectional" is a bit of a misnomer when it comes to aviation. An "omnidirectional" antenna radiates equally in azimuth (i.e. every direction on the ground), but radiates less in the vertical direction than horizontally. While this is perfect for ground communications that don't want to waste energy transmitting into the air, it's obviously less ideal for ground-to-air communications.
Analog transmissions don't strictly have a bit rate. They do have a bandwidth (of roughly 8 kHz), which very roughly corresponds to somewhere in the ballpark of 100 kbps.
The power is laughably low compared to many transmissions. A few tens of watts is plenty since the transmissions just need to go through empty air. By comparison, there are FM radio stations (which are VHF transmissions just like airband communications) that transmit as high as 320,000 watts effective radiated power!
Tower transmissions can go up pretty high. The transition between tower and other controllers is done by instructing the pilots to change to a different frequency, not by trying to make the transmission ranges exactly right.

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Are taller landing gear bad for aircraft, particulary large airliners?
Buiding larger engines for larger aircraft as opposed to smaller but many engines has proved to result in more fuel efficiency.
Having established one of the main drawbacks or impediments to building larger engines are ground clearance for engines in light of aircraft/airliners in their current under wing configuration. This configuration in light of current technological advancements seems to offer the greatest advantage in terms of control of the aircraft, stability and weight closest to the center of gravity when flying among other things.
If space to store the retracted gear were not a problem due to some new design, are taller gear for airliners necessarily undesirable for landing and take off? Do they present some technical problem if for example they were strengthened, the extra length would present some problematic leverage problems as they get closer to the pivot and "load"?

Tall landing gear has been used in the past:

The Tu-114 needed really tall gear to clear the really big props. The drawbacks are extra weight and volume, as described in the other answers.
The Tu-114 was so tall, on its first flight to the US (the prototype was used to ferry Nikita Kruschev to the UN for some shoe banging) the available boarding stairs weren't tall enough. That arrival raised some eyebrows in the West because they'd done a nonstop flight from Moscow (the Tu-114 was the first aircraft that made that possible).

How did they eventually get him down?

You can see that in the second photo. They placed a smaller set of stairs on top of the large one.

OK otherwise great answer and thanks, much appreciated

The Tu-114, itself, was a case of lowering what was a high wing on the original Tu-95.   Also the Tu-114 sorely needs a 4-pack of Progress D-27 engines; that would make it the coolest thing to ever fly.

... shoe banging?

@AzorAhai https://en.wikipedia.org/wiki/Shoe-banging_incident

It's just more weight and volume.
The problem really arises when you modify an existing airplane with a new engine that requires more clearance. Changing the engine is not that big a deal (relatively speaking) when it is slung below the wing as its physical interface is through the pylon. It is an appendage.
The landing gear are smack, dab in the middle of the fuselage, and changes in that will ripple through far more systems. It is literally right in the middle of things.

@CrossRoads It's the flat bottom on the nacelle that led to the pitch-up moment at high AoA. Having the weight of the engines farther forward would result in a pitch down moment.

@CrossRoads The engines on the 737 MAX aren't that much farther forward than on the NGs. They are higher, but not much farther forward at all. This is an NG (-800) and this is a MAX 8. There's currently a lot of misinformation and speculation spreading around in regards to the MAX, though. That's why we ban accident speculation here and wait for the official reports.

Changing the engine can be a very big deal if it affects the wing's flutter stability margin. Boeing had a terrible time trying the design the original B747 wing to be compatible with engines from the three manufacturers, with maximum commonality of parts.

@reirab if the flat bottom of the nacelles leads to pitch up at high AOA this can be solved by adding area to the H stab, just like they did with the canarded Ascender years ago.  I cringed when the MD-11 downsized theirs. A big one adds safety, and offers an option other than trying to fly the plane with a rapidly pitching trim function.

@RobertDiGiovanni The system isn't designed to fly the plane with it. It's designed to activate as the AoA nears critical to prevent stall, just like the almost identical system on every Airbus since the A320.

@alephzero True, but the MAX was designed for only one engine. They switched from the CFM56 to the CFM LEAP, just like the A320. The only real difference is that the nacelle has a flattened area on the bottom on the 737 (on both the MAX and the NGs) due to the low ground clearance on the 737. The 737 was originally designed for turbojet engines, which don't have anywhere near the diameter of modern high-bypass turbofans.

I have no experience in aviation but just from physics, longer landing gear "legs" (I don't know the terminology) would mean that as the aircraft lands and experiences a force perpendicular to the landing gear, the torque on the landing gear would be higher. This could present a challenge in that the structural stability of the landing gear "leg" may be compromised from increased torque, especially if we're talking about a heavier aircraft.

You should add figures to illustrate your answer.

Another way to explain this is: a longer lever sticking out from the plane means it has to be stronger to not snap off in a crabbed landing, or during braking, or any other sources of non-vertical force.  So yes, the landing gear has to scale up in thickness as well as length, so the weight required might scale as length^3 or at least length^2, not just linearly with length.  (The question does already guess that this might be a problem, but still +1.)

Taller landing gear do present challenges due to increases in leverage and greater amount of space, but this has already been dealt with in large delta designs such as the XB-70 and is certainly not insurmountable.
However, in the vast database of aeronautical endeavor there are solutions worth reviewing (wing? ah, review wing). Yes, many high wing designs would comfortably fit the larger engines.
Secondly, the trend towards fewer wing mounted engines has deprived air craft designers of a key safety feature from the tri-jet, using thrust to push the nose down.  Nose tractors (prop) have it as down thrust, rear pushers (prop or jet) have it as tail up thrust. Add power nose down.
So let's look at the A-10 Warthog.  Why not put the two large fan jets back there with future airliners!
Notice many major airliner designs trace unbroken lineage all the way back to the Me-262.
Every time I see one, I think this is the first jet airliner ever.  A great plane indeed.
But, with the development of more efficient and larger fan jets, designs such as the Ba-146 and many high winged transport aircraft can be brought into consideration to keep the thrust line in a manageable, safe, and even helpful configuration for wing mounts, if we wish to keep them there.

Engines on the tail have the disadvantage of longer fuel lines. Fuel needs to be transported from the tanks to the engines and when those are not mounted below the wings, fuel actually needs to be pumped to the back and up to the engines instead of more or less straight down from the tanks. Under wing engines have the shortest fuel lines from almost anywhere on the plane.

Why is thrust pushing the nose down a safety feature? My intuition from model aircraft would be that you'd want the nose to drop if the engines cut out, not under thrust.

You have it both ways.  If you cut power the aircraft slows, sinks, and (properly designed) H stab pushes nose down.  Thrust pushing nose down would also help, as does forward set CG!  But with tractor props, torque makes this a questionable option at low airspeed. Rear jet thrust, at an angle, not only pushes nose down to help gain air speed, it also reduces trim needs at cruise.

"Notice many major airliner designs trace unbroken lineage all the way back to the Me-262." That really seems like quite a stretch. The Me-262 is a fighter and has little resemblance to airliners of any sort, aside from wing-mounted turbine engines.

@Adwaenyth that ignores the fact that many airlines already have fuel pipes to the rear of the aircraft, because many airlines take advantage of using fuel as trim ballast by pumping it to tanks in the tail.  The real disadvantage to engines at the rear is weight - the entire tail structure had to be strengthened significantly, while the wings and centre wing box are already a strengthened structure for bearing the weight of the aircraft, so additional strengthening for bearing the engine weight and thrust adds little extra weight.

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Do airliners like Boeing and Airbus use airfoil variation along the span for their wings?
Airfoil variation will make a significant difference in airflow over the wing, thus proper variation according to the lift profile should improve the overall aerodynamic performance. But by how much? Are major aviation manufacturers using it? I can't find any reliable information on this.

Every good design varies at least the thickness: While the root needs to be thicker to enclose an efficient spar, the tip can be optimised for aerodynamics, which usually results in a root thickness of 16% to 21% and a tip thickness between 9% and 13%. Look at Dave Lednicer's incomplete guide to airfoil usage for an exhaustive collection of aircraft and their respective root and tip airfoils. Generally, larger aircraft tend to use thicker airfoils because scaling laws dictate that their structure has to carry higher loads.
Transsonic designs like airliners need to pay special attention to thickness because a thicker airfoil will encounter local supersonic flow at lower flight Mach numbers. Also, a thicker wing will store more fuel and will be lighter, so the compromise is around 13% thickness on average. With supercritical airfoils, a root thickness of 15% and a tip thickness of 11% has become widely used. Unfortunately, the exact airfoils of modern airliners are not published.
Another reason for varying airfoils is wing sweep. Left with the same airfoil over span, the isobars on the surface would show less sweep, creating a steep pressure recovery at the center wing (with potential for early stall) and suction peaks near the leading edge at the tips. Such an isobar pattern will use the available wing area less efficiently, so designers strive to make the isobars parallel to the local sweep angle by adjusting the local airfoils. At the root the maximum thickness must be shifted forward while tip airfoils need a more backward maximum thickness. Also, camber near the root is reduced up to the point where negative camber along the mid section of the airfoil chord is used.
Here are a few older ones:
Type                        Root airfoil           Tip airfoil
Boeing 377 Stratocruiser    Boeing 117 (22%)       Boeing 117 (9%)
Douglas DC-7                NACA 23016 (16%)       NACA 23012 (12%)
Lockheed 749 Constellation  NACA 23018 (18%)       NACA 4412 (12%)
Lockheed 385 L-1011 TriStar  ? 12.4%                ? 9%
Saab 340                    NASA MS(1)-0316 (16%)  NASA MS(1)-0312 (12%)
Tupolev Tu-104              PR-1-10S-9 (15.7%)     PR-1-10S-9 (12%)
Vickers VC-10               Pearcey 13%            Pearcey 9.75%
VFW 614                     NACA 63A015 (15%)      NACA 65A012 (12%)

Modern CFD techniques and composite structures allow to optimise the local contour at the wing-fuselage intersection such that no single root airfoil is used but a continuous variation which delays local flow separation and shock strength as much as possible. This also means that no stock airfoil is used, and most modern designs have their custom-made airfoils.

Sure the major aircraft companies are using airfoil variation along the wing span. There are a number of considerations in deciding the airfoil sections and their change when it comes to aircraft like its effect on the formation of shock waves (important for transonic airliners), the weight and cost considerations (in designing and manufacturing the wing with different sections). Most of the large aircraft use airfoil variation along the span; exceptions to this are smaller (GA) aircraft, though not all (and smaller rotor blades).
A good source of different airfoils used in aircraft is the University of Illinois Urbana-Champaign Airfoil Coordinates database and also airfoiltools. As an example, airfoil tools lists the airfoils used in the Boeing 737:
Root: Max thickness 15.4% at 19.6% chord; Max camber 0.2% at 5% chord
Midspan: Max thickness 12.5% at 29.7% chord; Max camber 0.8% at 10% chord
Tip: Max thickness 10.8% at 40% chord; Max camber 1.6% at 20% chord
You can see that the thickness reduces as we go out along the span while the camber increases. Basically, this is to optimize weight (thicker spar at root) and aerodynamics performance (thinner airfoils are better at higher altitudes and higher speeds).
Note: Most modern aircraft by large companies (like Boeing and Airbus) use in-house airfoils that are developed specifically for their projects and are confidential information; it is highly doubtful if that will be made available. But with the availability of modern CFD codes and manufacturing techniques (especially composite), we can be sure that the airfoil sections are being varied across the span.


text:
Why would METARs and ATIS both report exactly the same wind direction?
I understand that METARs report wind direction as true and ATIS reports it as magnetic. But at Camarillo (KCMA) both are reporting the same wind direction and speed even though the magnetic variation is 12.5 degrees. Why does the written versus spoken rule not apply here?

Are you comparing reports recorded at the same time?

Related: http://aviation.stackexchange.com/questions/60/when-are-winds-given-with-respect-to-true-vs-magnetic-north and http://aviation.stackexchange.com/questions/29643/why-do-metars-report-wind-relative-to-true-north-rather-than-magnetic

Written vs. spoken rule is not a rule, at least not that I can find tracable to any official source.  See AC 00-45G.

Here’s the info for ASOS.

VOICE MESSAGES
ASOS computer-generated voice message are made
available through telephone dial-in access and are broadcast directly
to pilots via ground-to-air radio (where installed). The information
contained in the ground-to-air radio broadcast message and the
telephone dial-in message are identical. The voice message is composed
of the body of the ASOS observation and selected remarks. Selected
remarks can include variable visibility, tower visibility, or Density
Altitude when 1,000 or more feet above airport elevation. Control
tower personnel may add Notice to Airmen (NOTAM) or other information.
Note that wind direction in the voice message is given in degrees
magnetic.

http://www.nws.noaa.gov/om/brochures/asosbook.shtml


text:
Where can I find an example of a propulsion maintenance task card?
I'm looking for an example of a task card for aircraft propulsion maintenance, e.g. propeller or engine maintenance. I've found examples for wings and airframes, but not for propulsion.

Why are you looking for such a thing (what underlying question do you have that a powerplant card could answer, but not an airframe card)?

This should be within the scope of Maintenance Manual.
The Job Task to perform are usually referenced to particular task of the engine in the Engine Maintenance Manual.


text:
Why do aircraft on a junk yard have blind windows (e.g. German "Landshut")?
Today I saw a picture of the “Landshut” (named after a German city, pronounced lund's hoot, not Land shut). The aircraft was hijacked in 1977 and now rusts at the airport of Fortaleza (Brazil) since 2008. German enthusiasts wanted to transfer the plane back to Germany as it somehow and sadly became part of German history, and the German government intends to do so as well.

(http://hessenschau.de/panorama/landshut-104~_t-1488105510615_v-16to9.jpg)
(Source)
I noticed the windows are blind. In an answer to an other question here I can also see a “retired” plane and Peter K. mentions the blind windows in a comment. This gave me the impression that all “retired” planes have blind windows.
Do the windows get blind simply because the planes are left on their own for years and wind, sun, etc. make them blind or are they made blind on purpose? If so, which purpose?

Addendum
After reading the good answers, I reviewed the video I already linked. This screenshot:

(Source: tagesthemen from Feb 23, 2017, 22:15h (German TV news magazine))
shows the cabin of the “Landshut” and it's clearly visible that the interior is removed and the windows are blanked from the inside and covered with grey foil. I just wanted to share this image.

They are put there on purpose, usually to protect the interior/windows from weather and sun. I'm trying to find an article on the process of mothballing an aircraft...

Possible duplicate: Why is this 777 with doors and windows covered?

Assuming you're more familiar with German (thanks for the pronunciation guide for Landshut!) than English, I'd use the term blanked instead of blind for the window coverings - even if they simply pulled down the built in blinds to cover them. /English.SE discussion

At Davis-Monthon AFB they go further then just blocking the windows. They encase entire aircraft in a tomb of "spraylat" to fight against the elements

@FreeMan You are right, I'm German and English is only my 2nd language. I believe you blanked is the better term, but to my understanding that would imply that somebody has done something (namely blanking the windows), and that was actually my question. If I'm wrong, feel free to improve and edit my post. And thanks for pointing this out. I always appreciate it when someone fixes my English.

No worries, @PerlDuck, you're one up on me! I know English and just enough of a few other languages to sound pretentious and unintelligent. I do know the German v->f and w->v pronunciations, but that's about it. I'm glad you took the suggestion in the helpful manner in which it was intended.

taken from Business jet news

For outside storage, cover windows and recognition/strobe lights with barrier material and pay attention to atmospheric conditions, as humidity will accelerate corrosion. Make sure all control locks or control column gust locks are installed to prevent movement of flight control surfaces.
  (emphasis mine)

One reason to protect the inside fabrics, plastics etc from the effects of discolouration from sunlight, as well as inhibit biological growth. Also without the coverings the temperature inside the aircraft will be significantly higher on sunny days see Desert Usa

[without protection] the interior could quickly heat up to 200 degrees Fahrenheit during hot summer days


text:
Is it more efficient to have many or a few rotors?
Wouldn't it be smarter to design VTOL rotorcraft with many smaller rotors than one big rotor? With more rotors you have more redundancy and smaller blade velocity (and perhaps better stabilisation) thus leading to a safer more reliable system. It seems like the problem with lots of rotors is that you lose all efficiency. I don't get this though. If a rotor prop produces x thrust at y efficiency, couldn't this be simply scaled up to produce the same thrust and efficiency ratio?

You should emphasize what information your didn't find  in the answers of the question Why haven't quadcopters been scaled up yet?

The answers are correct that multiple small rotors will be less efficient than one rotor, but the unspoken question of "why is this aircraft designed this way?" is that it's an electric tiltrotor--it uses differential thrusts and torques for control in vertical mode (mechanically simpler than traditional helicopter controls) and shuts off some of the motors in horizontal cruise.  I don't know what the predicted overall system efficiency is, but it can be better for some mission profile than a main+tail rotor helicopter even if the prop vs. rotor efficiency is strictly lower.

Related: Why do solar planes have many small propellers instead of fewer large ones?

Rotors need something to power them and the current generation of aircraft (once you get bigger than model size) burn hydrocarbons to get that power. If you want twelve rotors, then you either need twelve engines or you need a smaller number of engines plus a bunch of heavy driveshafts and gearboxes.
I assume from the fact that most contemporary aircraft designs use a small number of engines (usually one or two; sometimes three or four) that having a few larger engines is generally more efficient than having more smaller engines. This seems to apply equally to jet engines, turboprops and piston engines.
It's possible that the tradeoffs will change if you move to electric motors. However, at the moment, batteries are so heavy that electric motors don't seem to be a practical option.

The problem with many propellers is that more of the propeller slipstream will flow around wings and tail. The higher local flow speed might be beneficial if you want blown flaps, but for regular flight and especially in hover it will increase viscous and pressure drag without any benefit. This will reduce overall efficiency when the number of propellers goes up.
Generally, one single, big item can be made more efficient than many small ones.

So the solution for this problem is to keep the props above or below the wings?

@DanielCaoili: No, the solution is to use fewer, larger props.


text:
Has a "fight" ever happened in the cockpit?
Somehow, a variation of How do long-haul pilots cope with boredom? .
Has it ever happened that the pilots started some small talk about politics, or football, or favourite rock band, or that one cheated with the other's SO, and the discussion degenerated in some strong arguments, if not a fight? Is there anything in place to prevent that (like, procedures on what you're supposed to talk about during free moments)?

In the 100+ years of aviation history?  Yeah, it's happened.

It certainly went far beyond a strong disagreement and didn't come out of small talk, but FedEx 705 was a very serious fight in the cockpit.

@ZachLipton One of my former managers jumpseated with Auburn Calloway about 2 weeks before that incident. Said he was really arrogant and made a couple of minor errors during the flight that the captain questioned.

"My StackOverflow is better than your StackOverflow" /sorry

I've never seen anything about pilots fighting during the flight but on Air India 611 they fought before the flight then proceeded to fly anyway. There are differing accounts of whether it was physical or just verbal. The captain claims he was punched for calling his older copilot "uncle."
On Express Jet 4453 apparently they got into such a heated argument that when they landed, the flight attendant made the passengers disembark. They continued on in a different plane with a different flight crew.
There are reports of a fight between pilots on a China United Airlines flight but finding out what actually happened will be difficult.
On JetBlue flight 191 the captain started ranting and raving to the point where flight attendants and passengers had to remove him and physically restrain him for the duration of the flight. There's video of the incident. He was charged with interfering with a flight crew but was found not guilty by reason of insanity.
And on Delta 2598 the pilots did an emergency landing when two flight attendants got into a brawl.

Flight attendants do seem to lose it from time to time: http://www.theblaze.com/stories/2012/03/09/i-am-not-responsible-for-crashing-this-plane-flight-attendant-escorted-off-plane-after-freakout/ Although to be fair, that was right after Chapt 11, and right when the AA had filed to cancel the union contracts.

That's a great amount of interesting stories, thanks! But apart from common sense, there are no regulations trying to prevent this kind of things from happening? (Maybe they won't make a difference anyhow, but...)

As you mention, a regulation wouldn't make any difference - people angry enough to fight are already ignoring the law, company regulations, social norms etc... one more rule wouldn't have any more effect than the others

@OSUZorba Chapter 11? Did AA file for bankruptcy?

I like the idea of regulations, though. I have a mental picture of two pilots getting into a brawl, and at the very last moment, one of them says, "hey, wait a minute, I totally forgot about FAR §91.42 'Two pilots shall not argue on the flight deck'", and they just sit down and resume smalltalk :-D

@falstro Yes in November of 2011.

@OSUZorba Interesting. Did not know that, thanks.

In addition to the examples TomMcW cites, the final report into Germanwings lists on pp 72-73 several related incidents. The list includes the JetBlue 191 mentioned by TomMcW. Similar events occurred on a Egypt Air flight in 1999 where the relief co-pilot flew the aircraft into terrain despite the captain trying to override his input (on a non-fly-by-wire aircraft), and on a Royal Air Maroc flight in 1996, and a Japan Airlines flight in 1982. In all of those cases, the cause was (or was suspected to be) a psychiatric problem rather than a simple argument.


text:
What are acceptable treatments for chronic pain that will not result in a loss of an FAA Medical?
Is there an acceptable treatment plan for neuropathic pain? Gabapentin is not an acceptable drug. Lyrica and Cevalla aren't approved. Is there a treatment that could even get a special issuance?

Ask a doctor who is at least familiar with aviation medicine. This site has a lot of certified pilot but not as many certified doctor. Maybe if you post where you live someone maybe able to suggest a doctor for you. Get well soon :)

maybe Acupuncture?

The only real answer here is to consult an AME. I searched on the AOPA forums (members only) and the information wasn't encouraging: all current (as of 2014, which was the more recent relevant post I found) neuropathy drugs are disqualifying. AOPA's list of approved drugs does show some approved drugs for pain relief, but it looks like it's all OTC stuff.
But, the usual medical disclaimer applies here: every case is different so the above may be irrelevant to you personally; I'm not a doctor and I don't know what I'm talking about anyway; only an AME can give you meaningful information.


text:
East-West and West-East flight times in relation to earth's rotation
If I were to fly direct from from East to West, and then in the same aircraft do the return trip from West to East, assuming there is no wind on the earth, would both journeys take the same amount of time? This is of interest because logic tells me that the Earth's rotation should make the return trip longer.

Engines are moving the aircraft related to the air around, the speed relative to the air will be the same. So the only parameter that can play a role is how the air is moving related to the ground. In your question you seem to state there is no wind. So the air is immobile related to the Earth. So the trips take the same time, the Earth rotation plays no role. If there was wind blowing in one direction, then it would shorten the flight time in that direction, whatever it is. This is why crossing the Atlantic using the jet streams takes less time from North America to Europe.

I think I just got this question. He is talking about the "apparent" time that has passed in the airplane compared to the actual time because the earth's rotation causes the timezones to shift with or against the airplane, causing an apparent shorter or longer flight when comparing local times to the actual flight time.

@SMSvonderTann The way that I read the question, the OP believes that the atmosphere does not rotate with the earth, which would allow the earth to spin beneath you and either help or hinder your progress across the ground depending on your direction of travel...

@Lnafziger I think this has to do more with perception of local time at the origin and destination airports than with actual wind and stuff like that.

@SMSvonderTann Well, since he asks if they will "take the same amount of time" and thinks that "the Earth's rotation should make the return trip longer", and he never mentions time zones, I don't really see it...  Maybe he will come on and clarify for us though.

@SMSvonderTann I agree with Lnafziger that this isn't about time zones, but absolute duration and not understanding that the atmosphere rotates with the earth.


text:
What is the difference between a volume-limited-payload and a maximum-zero-fuel-weight-limited-payload?
Reading [Long Range Aircraft-AEA Requirements].
In appendix, Para 1.2 Definitions and specifications, the definition for "Payload" states

Payload should be the Volume Limited Payload (...) or the Maximum Zero Fuel Weight limited payload, whichever is the lesser.

How can we can define and understand "Maximum Zero Fuel Weight limited payload"?

In short, the Max Zero Fuel Weight payload will be that which brings the total aircraft weight up to that limit. The Max Zero Fuel Weight limit is a structural limit to prevent damage to the wing spars. No time for a more thorough answer at this time but I'm sure someone will pipe in soon!

Though I have not heard the term Volume Limited Payload before, I suspect they're talking about a passenger airplane. In pax aircraft you typically run out of empty seats before you bump up against the MZFW, In freighters you fill the air above each passenger with cargo and are often limited by the MZFW.

The Maximum Zero Fuel Weight Payload, is the amount of payload $m_{pl}$ (i.e.: passengers, cargo, but not fuel) required to reach the  Maximum Zero Fuel Weight condition while keeping the fuel weight, $m_{fuel}$ to zeros so:
$$
while \; m_{fuel} =0: \\
MZFW = OEW + m_{pl} \\
$$
The Maximum Zero Fuel Weight ($MZFW$) is an indicator of the maximum loads that the wing structure can bear. For the wing an "unloaded" situation (no fuel in the wing) is critical, since the fuel stored in the wing helps reducing the loads on it in flight. The lift acts "upward" while fuel weight acts "downward", thus the fuel contribution helps relieving loads from the wing structure.
The volume limited payload is an indicator for the sizing of the cargo compartment structures. These are the structures of the cargo area, where the LD containers are stored (under the floor of the passenger cabin). This Volume Limited Payload is set to avoid having too "dense" LD containers, the limit of density for the containers is around 350 $kg/m^3$, you can find some technical specifications for LD container here.


text:
Is it possible to start the engines while on push back?
I was recently on a flight and the the engines started after push back has been initiated. I was always under the impression that the process required a GAU to feed pressurised air or use of the APU.
But the APU wouldn't be used at a major London airport when ground services are available?
How did they start then?

"But the APU wouldn't be used at a major London airport when ground services are available" Why do you believe that? The APU is used to power the aircraft systems and air conditioning while on the ground, so it will probably be running anyway. Typically the bleed air from the APU is used unless the APU is inop.

Aircraft engines are started using the APU or a Airstart Cart. The GPU is for electrical power only and the engines are started off air power. It may be a Airstart was not available or they were running late on the block time and they blocked  using the APU to start the engines to make the block time. After engine start and before takeoff the crew will shut down the APU. Most US airlines start their engine off APU air unless the APU is not working then they will start off a Airstart Cart.


text:
Where can I get aviation weather information?
Where can I get weather information (preferably something similar to what pilots get) for a non-pilot?
I usually use flightradar24 for flight tracking, would love to see weather data as well.

METAR data are used for aviation activities. E.g. Atlanta or this format.

https://www.aviationweather.gov/adds/

Which countries or regions are you interested in?

NOAA collects METARs (current weather) from all over the world and publishes them at http://weather.noaa.gov/pub/data/observations/metar/stations/.
They also collect TAFs (forecasts) and publish them at http://weather.noaa.gov/pub/data/forecasts/taf/stations/.
These reports are what the information told to pilots by controllers or broadcast in ATIS is based on, written in shorthand format designed for efficient transmission over teletype. If you look around the directory structure, there are also decoded versions.
The above links are complete, but intended more for machine consumption, being plain text files. You'll find many other sources that look more like normal web pages if you google for some airport code and “METAR” or “TAF”, but different parts of the world will be available at different places. The above links are most comprehensive source.

The Aviation Digital Data Service (ADDS) is an excellent source for aviation weather information within the US. This website is free and completely open to non-pilots (as opposed to 1800wxbrief, for example).
The METAR page gives up to date weather observations for airports throughout the US. The other pages include many other useful weather products, including forecasts, pilot reports, and satellite or radar images.
As a pilot, this is a website that I use frequently, and have used throughout my career. To a non-pilot some of the products may seem incomprehensible, but the website does provide help pages. Perhaps most importantly for a non-pilot, many of the coded textual products are available in a decoded, normal language format.

Since you are asking as a non-pilot, I would highly recommend this particular site that I use for every single flight that I make, it is Sky Vector.Not only does it provide METARS and TAFs, it provides graphical depictions of:
TFRs
DROTAMs PIREPs SIGMETs AIRMETs
It also provides graphical depictions of MRMS Weather Radar, IR4 Satellite and Cloud Top Heights. Besides all of the weather services, you can plot a flight plan and overlay enroute hi and low IFR or VFR charts as well as selected sectionals.
I have sent several of my simulator-only pilot friends to this website for fast, accurate and up-to-date weather information and flight planning. FOr non-pilots it has all of the same weather information that we pilots use but in a nice, graphical format that is easy to readn and understand.

You can also search in your state, or a specific airport using this site
https://www.faa.gov/air_traffic/#weatherWidgetContainer
It pulls up a map and a list of airports and their automated info type, example of the first 3 of 20 entries for MA
Automated Weather Observing System  (AWOS)
Automated Surface Observation System (ASOS)
https://en.wikipedia.org/wiki/Automated_airport_weather_station
KBED    Bedford Middlesex   MA  ATIS - 124.6    (781) 274-9733  ASOS
KBVY    Beverly Essex   MA  ATIS - 119.2    (978) 921-5042  ASOS
KBOS    Boston  Suffolk MA  ATIS - 127.875  (617) 567-5762  ASOS
I carry the list of numbers for my state in my flight bag so I can call from on the road and see what conditions are like nearby, along with a call to Flight Service (1-800-weather) for the broad picture and NOTAMS, etc.

You might want to explain what the ASOS is since this question focuses on info for non-pilots.


text:
What are the popular algorithms or softwares used in identifying or track flights by ATC?
Just curious to know about the algorithms /software used by ATC for tracking flights in real time.Can it be classified under ERP ?If no under which category does that fall ?

Why would you need an algorithm to track a flight? Scheduling them, routing them maybe, but tracking them?

@RonBeyer If for no other reason than combining or fusing radar tracks from multiple sites. There are different algorithms in using with different pros and cons.

True, although with Nexgen track fusion is going to become less common. The aircraft are self-reporting with high accuracy GPS positions, track and heading, etc.

ARTAS. http://www.eurocontrol.int/artas

Software-wise, the big firms of the ATC industry such as Thales, or Indra have systems installed for a lot of ANSP, that enable ATCO to perform their duty. But the entire system needs a lot more than just a software or a few algorithm, since it connects radars, database and telecommunication elements.

@RonBeyer Any computer program is an implementation of some algorithm.

Can you please be a bit more specific? Algorithms/software is broad and ATC is using a trackload of software for many things. So it's even broader. Are you looking for how flight data are processed? How data are displayed? Please specify otherwise I'm afraid the question will be closed as "too broad" or even "off topic"

@DavidRicherby I'm not sure I (2 years ago) said anything about what an algorithm is, only questioned why they would need one in this context... I'm a a professional software developer myself...

@RonBeyer You need an algorithm because, if a computer does it, it does it with an algorithm. Even if it's a boring, obvious algorithm that doesn't have a name -- "Loop through the list of radar contacts and plot them on the screen" is an algorithm. Anyway, not a big deal.


text:
Is it possible to integrate an AI CoPilot on existing planes?
Or a new aircraft would have to be designed for such a thing? I have little knowledge of aviation, so please feel free to fill me in the required bits.

What is an "AI Copilot"? And how does it differ from a regular autopilot?

To make decisions for which you need a human inside the cockpit. It would turn the human pilot's role largely supervisory. It would be, as Canuk said, acting on its own high-probability inferences.

I think the bigger question is if we can safely integrate the already working software used in Unmanned Aircraft Systems/Aerial Vehicles into the Commercial sector. Obviously we would not literally use the same software, but the ideas and methods that have been proven to work.

I understand your question as: "do we have yet the required interfaces for a computer to fully interact with an aircraft". This is indeed the prerequisite before any intelligent system can pilot an aircraft.
That means could we yet plug a system into some bus to receive all signals available to a human pilot and send all commands a human pilot can issue.
The answer is no (e.g. a computer cannot lower the gears or receive electronic signals from the wet compass), but the required updates are not so large and involve limited retrofitting.
Of course, we don't have this computer yet, but this kind of software is not a huge step either (and anyway this is not in the scope of your question).
Would such system be economically profitable for an airline in term of safety, certification, operational cost, software maintenance, compared to a human first officer? If the answer is yes, then this is a matter of maybe 10 years before a full electronic pilot is tested. I doubt the answer is yes, it could be useful, but it will have an additional cost, so there could be a long time before we see it in a cockpit.

Aside from lowering the gear (which has been done, the Russian Space Shuttle Buran comes to mind, since at least the 1970's), a full auto-landing is conducted regularly. What we don't have yet is ground control (taxi in/out) and auto-take-off.

@RonBeyer Indeed, I am shocked to learn that lowering the landing gear is still not automatic, and one of the risks of flying is a gear-up landing: https://en.wikipedia.org/wiki/Belly_landing

@RonBeyer: Starting with STS-121, each US orbiter had also a  RCO IFM cable on-board. This cable could be used to connect different systems, so that commands for a complete automatic landing could be sent from the Mission Command Center in Houston.

@mins Interesting, didn't know that the orbiter had that, but it doesn't look like it was ever used, and I'm not sure that it is an autonomous system (requires operators in Houston). The Boeing X-37 says that it is the second spacecraft with auto-land capabilities after the Buran.

@DepressedDaniel Piper used to have an auto-extend feature. See http://www.planeandpilotmag.com/article/20-things-you-never-knew-about-your-airplane-or-someone-elses/#.WO3SkIjyuUk, item 4, for why they discontinued it. If I bought an airplane with the auto-extend feature, I would disable it. The decision  as to when it's best to drop the gear is not always a straight forward one. It's a matter of opinion, but mine is that I'm willing to trust my ability to get the gear down rather than forego flexibility.

If an "AI Co-pilot" existed, it could likely be integrated on existing airplanes with an auto-pilot system.
Since your question is a little vague, I'm going to take a stab at some your hidden assumptions and go from there. I'm also making the assumption that you're talking about the enroute phase of flight, and leaving the taxiing, take-off and landing in the hands of the human pilot.
At the risk of (extreme) oversimplifying, an AI system is essentially something that takes in a lot of data, analyzes it (through different algorithms) and outputs data that can then be used to do something.
I'm going to assume that the AI algorithm is supervised and has been trained on a bunch of existing data, and it has been figured out what data is important to make high-probability inferences.
In-flight, there are multiple sources of data that could be fed to the AI algorithm for it to make probabilistic inferences on what it should do.  These data sources are things like:

Engine monitors (my EDM 700 I think logs 60 data points/second on things such as cylinder head temperature, exhaust gas temperature, RPM, location, power setting, etc.)
GPS Receivers (3D position)
AHRS (attitude information)
ADS-B (Traffic and Weather)
On-board Weather Radar
XM Satellite Weather

(As an aside, SavvyAnalysis already does a bit of this type of analysis using the data from your engine monitor).
So, provided that your "AI Co-pilot" has been properly trained in advance on the same type of data that you are able get from your airplane while in-flight, your AI Co-pilot could ingest the data, and output signals to the auto-pilot which would then fly the airplane. The autopilot controller is simply sending information to a particular servo for a particular control surface (aileron, elevator, etc.) and if your AI Co-pilot can output the same signal, then it could control the servos the same as the autopilot.
One of the (many) difficulties lies in designing the system to safely handle things that it hasn't seen in the training data.
But to answer your question, it wouldn't require the design of a new aircraft, but rather a new autopilot control system.
EDIT: Extra thoughts
After thinking about your question some more, I was thinking why you might want an AI Co-pilot? I'm thinking the answer to that might would be for it to either make decisions for you that you are unaware you should be making or to help you make better decisions.  There is existing software that does something like this, called Xavion, which continually figures out where you should land if your engine quits based on the current location and performance of your airplane. It's not hooked up to an autopilot, but it wouldn't be too far of a stretch to think that sometime in the future, something like this could be a part of the autopilot controller.

Furthermore, Xavion is constantly running engine-out flight
  simulations behind-the-scenes as you fly. The app imagines an engine
  failure every single second, and it checks how your plane would do in
  a power-off glides to every runway at every airport in gliding range.
For each of these simulated engine failures, Xavion estimates the
  likelihood of a successful landing based on the runway’s length,
  width, proximity to the aircraft, and glide path from your current
  location down to the threshold of that runway. When Xavion has found
  the runway that it believes is most likely to result in a safe
  engine-out landing, it builds a series of hoops from your current
  location to that runway—spiraling, circling, or extending as needed in
  order for the hoops to follow a constant descent angle that your
  airplane should be able to sustain without power.

When you start to think of something like this, you can start to imagine analyzing real-time weather data and having the system make (or help make) a decision to divert or land based on your performance and experience. However, this is all something that would happen in the auto-pilot controller, so retrofitting it to an airplane with an existing autopilot would be more of a software upgrade than a hardware redesign.
UPDATE:
Wired had a good article on this on 3/28/2017. It talks about all the different sources of data that could be used to make an AI learn about the airplane, figure out where the holes are in the data and who could fill them.  An interesting place for data that I didn't think about (because it's not data generated in flight), is maintenance and service logs.
Particularly relevant to the intent (I think) of your question might be this passage:

Elsewhere, researchers are working to ensure AI can help pilots manage
  crises as they arise. At University College London, a team led by
  Haitham Baomar and Peter Bentley is developing a new autopilot system
  that learns how to manage emergencies by watching how well-trained
  pilots do so, and then behaving as they do in similar circumstances.
“We want to increase safety by trying to tackle the human-error factor
  that might be caused by stress, information overload, and sometimes a
  lack of sufficient and up-to-date training,” Baomar says. “Modern
  autopilots, unfortunately, can’t handle challenging flight conditions
  such as severe weather conditions or system failures.”

Anecdote comment: First time subway come full auto people are scared of boarding a train without a conductor. For the airliners co. a AI Copilot can cause some marketing concerns

@jean I think people might have less problems with a system where an actual human pilot is still in the cockpit and able to take over at a moment's notice should something go wrong, than with a full-automatic system with no human pilot with "skin in the game". I think we have a question somewhere on completely automated passenger flights which discusses some of those issues; a case could probably be made that combining the good parts of technology and human pilots make for an overall improvement. Of course, that doesn't address the similarities between OP's proposal and eg modern autopilots.

@MichaelKjörling I believe that this is the questions that you are referring to: Why do we still use pilots to fly airplanes?

@Lnafziger Yes, that looks like it's the one. Thanks!

That Xavion software sounds like a tremendous asset! It looks to solve a problem that seems to be happening with decreasing frequency, but I can see where it would be a great benefit to the pilots to recognize that they have an engine out, push a button, and a prepared solution pops up instantly. While pilots/ATC are cool as a cucumber, it would take away a lot of the stress and increase the likelihood of a safe outcome from an engine out situation.

We already have this - it's called the autopilot.
The autopilot has gone through continuous improvement.  Modern ones are hooked up to GPS, can control the throttles for optimum fuel efficiency, and so on.  They're a very sophisticated bit of kit.
More recently, autoland is standard on large aircraft, and is becoming more common on smaller passenger aircraft.  Again, these vary in sophistication, up to full "zero-zero" landing completely under the autopilot's control.

Same for military planes too?

"More recently, autoland is standard on large aircraft" Recently as in the late 60's and early 70's? Autoland has been around for more than 50 years.

@RonBeyer More recently than the original autopilot. :)  And it became standard more recently than that.

An autopilot won't abort the landing if there is a runway incursion. A human will do it, and that's what is expected from an intelligent autopilot. Intelligent autopilot is as different from autopilot than cruise control is different from a self driving car.

@mins It's not exactly different - it's adding extra features.  Cruise control does some of what a regular driver does.  Adaptive cruise control does a bit more.  Lane assist, ABS, active collision detection, auto-park, all those things do a bit more, and it'll be the same for autopilots too.  It's entirely possible to break the task down into chunks and solve each one in turn, so your AI copilot becomes progressively more helpful.  The eventual goal should be full automated control, because a properly-programmed system will be more reliable than a human being, but we're still getting there.

@mins On cruise control, its state is based on current inputs but also on previous inputs and previous states, so your first statement is untrue.  For an autopilot, all that's needed to reduce the pilot to a supervisory role (which is what the OP specifically asks for) is full control over all phases of flight.  The learning phase MUST happen during development only, because that's the only way this is safe.  Once it's installed, there's no more learning needed or wanted.

@Graham: As soon as you switch to AI (and that's a big debate today), you can't think the same way than with deterministic automation. If you learn from experience to improve as do humans, then "The learning phase MUST happen during development only, because that's the only way this is safe" is indeed a thing of the past. Cf. this case for permanent learning (permanent addition to corpus of knowledge).

@mins My point is that this is not a thing of the past.  When it comes to carrying out a narrowly-focussed task repeatably and reliably, this is precisely what you want.  There are well-bounded areas where the system should learn, for example how much rudder control is necessary to compensate for cross-winds, but these are easily covered by well-defined strategies such as Kalman filters.  Your links cite examples of learning which have no relevance to this task, and applying these principles in a situation where they are actively dangerous would be gross incompetence and must not be used.

@Graham maybe I'm misreading you, but the equivalent to your "The learning phase MUST happen during development only, because that's the only way this is safe." statement is equivalent to "The pilot must only use what he learned in the classroom because those are the correct theories, anything else is unsafe." I'm not a pilot, but everything I've read about flying (and everything else in life) is that theories are good but practical experience is far better. An AI begins with a set of assumptions but learns from experience to make it better, just as humans do-hence artificial intelligence.

@FreeMan That's a lot of assumptions though.  For starters, there's a reason pilots are type-certified, which is that randomly picking it up as you go is unsafe.  Instead you have a well-defined period of learning, after which you know what you can and can't do.  Similar, yes?  Then whatever practical skills you pick up are as a result of making mistakes and correcting them (or having someone feed back to you); but an AI has no way of knowing what constitutes a mistake, and there's no-one on the plane who can teach it, so it is fundamentally unable to learn that way.

@FreeMan But more than that, what automation brings to the party is not flexibility, it's 100% rock-solid deterministic behaviour.  A large proportion of incidents are due to pilot error, and that randomness is precisely what you do NOT want your autopilot to replicate.  More than that, there are incidents where something fails and the pilot doesn't know how to recover.  The autopilot can have those scenarios pre-programmed.  So that kind of learning really isn't needed or wanted.  It doesn't have to be perfect; but by being 100% reliable and not making it up on the fly, it's better.

@Graham: In case you want to know more about the current change in automation: deep learning, and how it can be used.

@mins Thank you for posting links which establish that what I'm saying is the case.  Both links state explicitly that an autopilot system would be be pre-taught a suite of scenarios based on existing data and existing situations.  As a software engineer who's spent roughly 15 of my 20 years' experience in safety-related engineering, I have some understanding of how utterly unsafe and illegal it would be to do anything else.


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What does this plus (cross) sign on this electrical panel mean?
I'm looking at a picture of the electrical panel of a Phenom 100 and on it I see a + sign, does anyone have any ideas as to what it's supposed to mean?

ymb1's answer is excellent, however I didn't fully understand what it meant until I saw this video.

I'm complementing @ymb1's answer, referencing an authoritative source.

Backlit frontplates (lightplates) embed LED which have to be connected to main circuit. The + sign indicates the location of the connector, in compliance with MIL-DTL-7788H specifications.

3.3.3.5.1 Connector location. A white, non-illuminated cross (+) approximately 0.08 inch (2.032 mm) by 0.08 inch (2.032 mm) shall be centered and located on the front surface of the panel approximately above the electrical connector (see figure 1).

To connect the front plate to its motherboard, the plate connector is aligned with the corresponding motherboard connector and fit by pressing the plate. Pressing elsewhere than behind the connector is less effective but more importantly can damage the pins, especially on small plates.
As @ymb1 mentioned earlier this is de facto where one can also fiddle with the connector in case of intermittent contact.


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What are these protuberances on the north edge of runway 10/28 at Mataveri?
The single runway (10/28) at Mataveri International Airport (IPC/SCIP) on Rapa Nui1 has (at least according to Google Maps) a few paved protuberances emanating from partway down its northern edge which serve no obvious purpose:

I wondered if at least some of them might be the old turnaround pads from before the runway was lengthened in the early 1980s, but:

if that were the case, one would not expect them to still exist in what at least appears to be good condition a third of a century later, and
they look far too small to be turnaround pads emeritus - compare, for instance, the turnaround pad at the approach end of runway 28:

Does anyone know what these appendages are, and what purpose they serve?

EDIT: As @MikeSowsun points out, the runway would not only have been shorter then, but narrower as well (not having needed to serve widebody jets or distressed spacecraft), meaning that the turnaround pads themselves could be smaller, and that they'd have been at least partly paved over when the runway was widened (leaving only the outboardmost portions visible); this still doesn't address the issue of "how come they're still visible a third of a century after being abandoned", though.

1: Also known by its colonial names of Easter Island and Isla de Pascua.

Some of the older aerial photos available in Google Earth show a painted turnaround arrow on the larger pad. Perhaps that's designed for smaller planes that need less space and a shorter runway.

If you look closely at the top circle of the top picture, you can see the nose wheel line for a turnaround pad.

I suspect they were probably old end of runway turn around pads from when the runway was shorter and narrower. As "user71659" says, I can see the nose wheel line still present on the larger one.

@MikeSowsun: Still doesn't explain how they still exist in fairly good condition, though.

They could still be available for smaller aircraft which don’t require full length takeoff. This would save taxi time, fuel and keep takeoff noise more inside the aerodrome area.

@Sean - "how come they're still visible a third of a century after being abandoned?"  Not sure what you would expect them to look like, but pavement takes a lot longer than 30-40 years to decompose.


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Are there any airliner crashes that can be definitively attributed to Mach tuck?
Mach tuck (the tendency of aircraft to develop an increasingly nose-down pitching moment at speeds above its critical mach number) caused a number of crashes of high-speed fighter aircraft before the phenomenon was well-understood, but I haven't been able to find any information regarding whether early jetliners (or modern ones, for that matter) were involved in any crashes that could be definitively attributed to mach tuck.
Were there any?

I've never heard of one, but note that the pitch down effect of Mach tuck starts  below Mcrit, and below Mmo for that matter, and airliner trim systems have a "Mach Trim" function to drive the stab to counteract it.
Fortunately, in the speed range of a commercial airliner, the amount of tuck is modest and it's not really a controlability issue, just an increase in trim speed.  Flying manually without Mach Trim, it would just require a manual trim input to keep the nose from dropping.  If the autopilot was on, it would take care of the trim change on its own.
So I would say that an airliner would never encounter a mach tuck condition that could cause loss of control unless it was way above Mmo in the first place, which means it was already out of control for some other reason.


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Why are piston engines so unreliable in airliners?
Before the first jetliners appeared, all airliners used piston (reciprocating) engines, which were astoundingly prone to failing in flight, to the point where an inflight engine failure was an everyday, expected occurrence.  To quote Wikipedia:

Engine failures were considered fairly routine events on piston-engined airliners in the 1940s, so the crew elected to continue the flight to Dallas, and Captain Claude announced to the passengers that they would switch to another airplane upon arrival.

In contrast, in-flight failures of modern turbofan engines are so rare that most pilots will go their entire careers without ever encountering one.  Even in the 1950s, jets were already more reliable than piston engines, which was one of the reasons they quickly seized centre stage for long-distance operations.  For low-altitude short-haul flights, where jets are inefficient compared to propellers, airliners still switched away from piston engines, moving to turboprops en masse despite piston engines having better fuel efficiency than turboprops.  All of this is despite turbine engines placing far greater thermal and mechanical stresses on their components than any piston engine.
Nowadays, piston engines are generally used only on very small general-aviation airplanes... and are no less reliable than the turboprops and jets on larger aircraft.
What is it about piston engines that makes them so unreliable on large aircraft, yet extremely reliable on small aircraft?

There are A LOT of moving parts in a two-row radial engine (R-1830). Some of the bigger engines had up to four rows (R-4360) but not many, if any, popular passenger planes used the biggest engines. With more moving parts, more opportunity for metal-on-metal wear, and the general complexity of the valves/turbos/carbs...there was a lot to go wrong. A turbine has fewer moving parts and creates MUCH more power for its weight. The monster -4360 made 4,300hp and weighed 3,800lbs dry. The Allison T56 makes roughly the same power and weighs less than 2,000lbs dry with fewer moving parts.

Plus any more than two rows of cylinders was extremely difficult to cool properly, which was the main reason for the problems with 3 and 4 row engines.

I think you're overlooking the great improvement in the reliability of all engines since the era of piston-engined airliners.  Compare a 1940s auto engine to one from the '60s, and to a modern one.  Then too, I think turbines actually place a lot less stress on their components.  Turbines basically just go around and around, while pistons, valves, and so on have to reverse direction a couple of thousand times every minute.

"Nowadays, piston engines are generally used only on very small general-aviation airplanes... and are no less reliable than the turboprops and jets on larger aircraft." - As someone who commutes daily in a single-engine piston airplane across an urban areas, I wish that were true. Light aircraft piston engines, including modern ones, have a comparatively terrible reliability record.

Nowadays you don't have to change the points on your distributor, that doesn't exist anymore, or clean the carburetor and change the fuel pump every 40k miles.

Ahh - Lockheed Constellation, the best three-engined airliner ever!

Compare quality assurance programs of the 40's to today . A jet engine of the 40's was less reliable than one of today and probably less reliable than a piston engine of  the 40's.

This answer does not need to be complex.  Piston engines have many more parts, and they are reciprocating parts.  They are much more complex machines than turbofans.  That's the answer.
Turbofans are fairly simple machines.  Yes, there are a lot of parts, but many of the parts are static, or move very little.  Their development and construction is complex, but their operation is relatively simple, and therefore more reliable.
For example; an 18 cylinder radial engine requires approximately 54,000 ignition events per minute of flight.  A turbofan requires only one ignition event for the entire flight, and it happens on the ground.
Similarly, that same radial requires two valve operations per ignition event.  That's 108,000 valve operations per minute, or about 19.4 million valve operations per engine on a 3 hour flight.
This means that a Douglas DC-7 would have 78 million valve operations on a 3 hour flight, and 39 million ignition events.  Ignition systems used to be largely mechanical, with many moving parts.  (Electronic ignition and distributorless ignition has changed this a lot in modern piston engines.)
Combine with this, the fact that they had devices like the turbo-compound, which you could think of as a turbo-shaft drive that helps drive the crankshaft.  Each engine on a DC-7 had three of them.  Incidentally, they caused reliability problems because they raised exhaust temperatures, and were known to cause exhaust valve failures.
3 hour DC-7 flight example at 350 mph: Denver to San Francisco
Reference: math, and engine knowledge
The Wright R-3350 on the DC-7 is geared at 16:7, or about 2.29:1.  Use 2,600 propeller RPM to get 6,000 engine RPM.

And, since a propliner flies more slowly than a jetliner, the same flight will take longer in the propliner, leaving more time during the flight for something to go wrong.

Great answer but I suspect the DC-7 used a much lower propeller rpm than 2600 - the tips would be supersonic at that speed.

there's no way an R-3350 could possibly run at 6000 RPM.

While the math may be slightly off, it does make the point. Even if it's off by 10% that's still a lot of valve openings/closings and spark plug firings.

This statement is not entirely true. the R2800 engine was considered at the time to be quite reliable in comparison to the R3350 and the R4360. Those engines represented the evolutionary dead-end of big piston power, where pushing to more power began making the engines less reliable than their lower-power predecessors had been.
Part of this was simple statistics, where the probability of failure of a machine scales with the number of parts it contains. In crude terms this means doubling the number of pistons doubles the opportunity for something to go wrong with one of them in the engine. Note also that those extra pistons do not provide redundancy, in the sense that a failed piston will fill the oil system with broken metal, causing the other pistons to fail not long after, or start the engine on fire.
Turbine power plants provide more uptime and longer TBO's than the piston engines they replaced as well as more power for less engine weight. These attributes offset the greater replacement costs and higher fuel consumption in applications where uptime and power were required to enable a viable business model.
Cost-sensitive, low power applications still make use of pistons, with the breakpoint being around ~300HP. This is at least partly due to the fact that as turbines are scaled down in power output, their efficiency usually suffers, and their manufacturing costs do not fall fast enough to compete with pistons in the 250HP arena.

One would think that the cascading-failure problem with big piston engines could easily be solved with a simple oil filter...

well, once the piston bits fall down into the crank shaft, it's game over...

It's not like turbines would be more cascade failure resistant.... :)

You in-part answer your own question

piston-engined airliners in the 1940

compared with

in-flight failures of modern turbofan engines

Simply put we have learned a lot in the past 75 years about manufacturing and construction, as well as how to test for, identify, and prevent fatigue. This knowledge has helped piston engines just as much as it has jets. As others have noted there are also far more moving parts in a piston engine that all have to operate in sync for everything to run.
Since piston engines are no longer used on the big aircraft the best analog is those that are still flying on GA airplanes. FAA analysis from the twenty years between '84 and '04 show that only 26% of accidents were mechanical and of those only 11% were reciprocating engine failures. In other words aviation piston engines are actually pretty reliable when used properly. Other trend reports note that failure is often related to misuse of the engine control system. In the 40's there was no FADEC, there may not have even been rev-limiters, operational parameters were based on decades of data not nearly a century.

...so unreliable on large aircraft, yet extremely reliable on small aircraft?

The piston engines on small aircraft are not by any means new. The Lycoming in the Archer I fly is pretty much the same as what it was when it came out in 1955. It's reliable because our machining, casting, manufacturing, and related functions are better than they were in 1955. A lot of people are flying around with full engine monitors which is a lot more than the single temp gauge the airframe originally had. A case could be made that an extra 50 years of development on large piston engines would have made them just as reliable as their GA counterparts.
Turbines are not without their issues either.

Another thing to consider is that a lot of the hanger talk (or what's left from that era) of large piston multi flying has heavy military roots. Engine servicing in the field was good but not great, and when heavy daily use, live fire, and not always "by the book" operations are accounted for reliable operation should never be expected. This has lead to a lot of stories of failed engines, gliding home (or into a field) etc. This helped to fuel the twin market for a long time and helped to create the notion that large piston engines were unreliable.

While it doesn't relate to reliability, operating piston engines at enough power at both high and low altitudes requires the use of superchargers (which add mass, decrease the efficiency, requires additional components, ...).
Also, not strictly related to reliability:
Hughes HK-1 'Spruce Goose'. Eight R-4360s making up a total of 448 spark plugs for one aircraft.
Avro Lancaster (4 engines of 12 cylinders, two spark plugs per cylinder). ... and it's real fun changing them. Mind you its not as bad as checking and re-setting the valve clearences, come on how many, 48 X 4 = 192 of the bloody things....

4 engines x 12 cylinders/engine x 2 spark plugs/cylinder = 96 spark plugs...


text:
Why can’t NEF items be placed on an MEL?
Why can’t Non-Essential Furnishings (NEF) items simply be incorporated into the Minimum Equipment List (MEL)? I understand that an NEF is part of an MEL but what limits the items on an NEF from being placed on the MEL itself and getting rid of the NEF all together? I have heard the MEL items are components and systems but have not found that verbiage myself. If we didn’t have an NEF list we would still be limited from flying with things that aren’t essential to the safety of flight. (I.e broken coffee pots, toilet paper holders etc.). So I understand that NEF items aren’t safety of flight but in terms of grounding an aircraft they’re just as important. So why can’t they just not be out in the MEL?  AC 91-67,Pl-34

I'm not 100% but I suspect: MEL is regulatory, NEF is purely company policy. So by keeping it separate, it makes it a lot easier for the company to take things off the NEF.

Also, the process for what you do when something on the main MEL is broken somewhere without maintenance facilities may be very different from if something on the NEF is broken.

This question would be greatly improved by explaining your acronyms and/or the context in which they're used.

User1937198- That seems like it would be the most plausible answer to me as well but why is the NEF part of the MEL then?

I think you kind of answered your own question.
Non-Essential Furnishings do not belong in an MEL because the presence or not of the items has no safety impact and therefore there is no safety analysis performed or required.
All of the items in the Master MEL that the OEM publishes have gone through a safety analysis to determine the risk associated with dispatch with a component or system inoperative, including follow-on failure effects, etc, to establish that a minimum safety level is maintained. As well there are maintenance and/or operational procedures associated with the MMEL items.  No such procedures needed for coffee pots.
So adding NEF items to the MMEL is pretty much pointless and a considerable amount of un-necessary hassle for the OEM.

Thank you so very very much John! That makes sense to me! I appreciate people like you who have vastly more knowledge than me taking the time to share insight and that knowledge with me!

Thanks.  It's worth noting that the sole purpose of the MMEL in the first place is to increase dispatch reliability.  When a type is certified, the OEM will spend the follow-on years trying to add items to expand it.  Each item added will add some small fraction to the airplane's dispatch reliability.  The industry DR standard is 99% of flights leave within 15 min.  A plane may have a DR of 97.7, and adding component or subsystem X to the MMEL might improve that to 97.8.  Eventually all the items add up, and you're there.

That’s one thing that I certainly had no idea of is dispatch reliability.  Thank you for that!

How does adding to the list improve reliability?  Unless it is replacing a less reliable component, mathematically the more items on your list the lower your probability that each will be 100%.

@MichaelHall Reliability refers to the fleet's dispatch reliability, the global on-time departure rate (on-time being leaving the gate within 15 min). An item added to the MEL turns failures that would cancel a flight into delays (say, removing a part and installing a blanking plate, which may take you past 15 in but at least you get away), and failures that would cause a 15min+ delay into on-time departures. A servo may break constantly, causing endless delays and cancellations, but adding it to the MEL,it still breaks all the time, but it doesn't result in departure delays/canx when it does.

In other words, adding it to the MEL makes it deferrable instead of grounding the aircraft, would that be a good way to summarize it?

Yes.The expansion of the MEL over the years as the design matures can raise the fleet DR as much or more than the actual reliability improvement mods. It also allows the operator to get by on fewer spares, and not have to provision out-stations with spares, when you can fly for, typically,10 days with a subsystem deactivated so you can schedule a visit to a maint base within the 10 days.  More safety critical subsystems you may only get 48 or 72 hour deferrals, whatever the safety analysis will allow, but you take what you can get. OEM will run a working group that just works on MEL expansion.


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What is the date of first use of the TAF format?
Unlike METAR and NOTAM, I cannot find any history on Terminal Aerodrome Forecast (TAF). Any hint as to the age of the format would be appreciated.

TAFs quite similar to what they are now were used with the rollout of DUATS in 1990.  Prior to that there were several transitions from teletype symbols to normal characters. Even with the old special tty symbols, the TAFs still had essentially the same content.


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How do I find out what happened on this flight?
On 2017-01-03 DAL58 from Boston to London apparently returned to New York after 90 minutes of flight.
This seems quite irregular to me, and I had expected to find details about this published somewhere. I tried Google, and I searched at avherald.com for the date and also for B763, but didn't find anything.
Was this not sufficiently irregular to warrant some kind of public information about what happened on this flight? Or if such information is published, where could I find it? Or is such information only available to certain people?

Call the airline and ask? Mind you, if you have no real reason for asking, they have no real reason for answering.

@J.Hougaard good point, indeed I don't have a good reason to ask - I'm just curious and thought there were public sources of information for this.

Not really. Just like there aren't public sources with information about car accidents, delayed trains, cancelled ferries and so on.

Can't find anything specific on that flight, but the drop in cruise altitude might indicate engine issues (but it could also just be that that's where ATC needed them for flying the opposite direction.)

Looking around on liveatc.net, I did see a couple of trans-Altantic flights that diverted to Shannon, Ireland (one for health emergency, one for unruly passenger) and a United flight from Sydney to SFO that diverted to Auckland due to an unruly passenger on that day, but I didn't see the Delta flight.

@reirab good point about liveatc.net, that's an interesting separate source.

If it was something critical like an engine failure, they would have diverted to an airport that was much closer. What likely happened was some sort of malfunction that meant they couldn't fly on to London but was not critical. They would have diverted to a place that would be most convenient for the passengers to get them on other flights, which they assessed to be JFK. This type of decision making is quite common and safe.

Issues in flight are more common than we think: engine shutting down in flight, fumes in cabin, ... Aircraft are huge pieces of engineering with many parts and sensors that can fail. Aircraft have many redundancies which most of the time allows for a safe landing.
I tried to look on the avherald, a reliable and extensive source of aircraft reports, incidents and accidents, mainly fed by its owner and its users but the flight does not appear there. However you can appreciate the number of incidents occurring every day and this is a small part of them.

This is all good points, don't forget medical emergencies as well.